Shinji Kagawa's best is yet to come for Manchester United, according to his former Borussia Dortmund team-mate Roman Weidenfeller.

Kagawa joined United from Dortmund for £17million last summer, but has struggled to reproduce the form he showed in the Bundesliga in his first season in the Premier League.

A knee injury sidelined Kagawa for two months prior to Christmas, although the Japan playmaker showed his class with a hat-trick in United's recent 4-0 win over Norwich.

And Dortmund keeper Weidenfeller, who played alongside Kagawa for two years with the current German champions, is convinced Kagawa will prove his worth in his second season at United.

"Shinji's difficult season is probably related to the injury he had, but now he's fit and back to his best form he can show what he can do," said Weidenfeller.

"He's proven he has a lot of potential and only a few weeks ago he scored three goals, so he is getting better all the time. He's still young, so there's a lot more to be expected of him and he will develop

"Shinji is a really good techical player, he has great technical skills and another great advantage of his as a player is that what he does is unexpected, so opponents find it hard to forsee what he's going to do, as he's so agile and moves quickly around the pitch.

"Another advantage Kagawa is that he is flexible in being able to play in different positions. At Dortmund he always played just off the main striker, but I wouldn't necessarily tie him down to just one position."

United have been linked with a summer move for another of Weidenfeller's Dortmund team-mates, the £20million-rated Poland striker Robert Lewandowski, with Sir Alex Ferguson a known admirer.

Lewandowski has scored 65 goals in 125 appearances for Dortmund, marking the 24-year-old out as one of Europe's most prolific strikers, with Weidenfeller offering a succinct verdict on why he is in such demand.

"It's a simple, straightforward and honest answer, but Robert scores goals - that's why he's so valuable to the club," said Weidenfeller.

"Of course, it's hard to hold on to really good, young players, but at the same time these young talents know what they have here at Dortmund, they know they're being supported well here, so there are also advantages to staying here too.

Although reigning Bundesliga champions Dortmund currently trail Bayern Munich by 20 points and have no hope of retaining their title, they have emerged as genuine contenders to win the Champions League.

Dortmund beat Real Madrid and Manchester City on their way to topping their group and United boss Ferguson believes Jurgen Klopp's ebullient and enterprising young side can be crowned European champions.

"It's a big honour that respected coaches have such a high opinion of Dortmund, but we can't get carried away by it.

"We need to concentrate, stay focused and see how we progress in the competition.

"The secret [of Dortmund's success] is to let young players develop and treat them with patience.

"The key is to let the youth develop at an appropriate pace, let them prepare for the Bundesliga and then the Champions League."

For the first time in 17 years there are no English teams in the last eight of the Champions League, althoough Weidenfeller does not think this is indicative of a drop in standards in the Premier League.

"It's a pity no English teams are left in the Champions League, especially United, as I would like to have faced my old team-mate Kagawa," said Weidenfeller.

"But it's hard to make a judgment on whether the lack of English clubs in the quarter-finals is part of a trend, because there are still some big teams in the Premier League.

"We will have to wait and see in the next couple of years what happens with English teams in the Champions League, so it's hard to make a judgment [on whether the Premier League has fallen behind the rest of Europe]."